Resistance to current flow occurs at the molecular level of substances. An electric conductor consists of atoms having free electrons in their outer most shells. These free electrons ordinarily move randomly from one atom to another. However, when voltage is applied across the conductor, free electrons flow from negative to positive charges.

Click on the slider dimmer switch to adjust current flow and the brightness of the light bulb filament.

The coil above represents a light bulb filament. Beneath it is a greatly magnified cross section of the filament showing atoms and electrons. Notice that as electrons move through the conductor, some collide with atoms and other electrons. It is these collisions that cause resistance. As electrons collide, the energy used to cause the current to flow is converted into heat and light.